This invention relates to a method and system for ultrasonic imaging, and more particularly to a method of ultrasonic imaging and a digital rectilinear real time ultrasonic imaging system capable of scanning rectilinearly which uses time delay focusing and does not require an impractically large number of multiplex switches.
The two most successful types of real time ultrasound systems, imaging at 30 frames per second or faster, are the Fresnel focused systems and the steered beam, phased array systems. The Fresnel approach is disclosed in U.S. Pat. No. 3,911,730 and in French Publication No. 2,292,978 by Pierre Alais, inventor, entitled "Improvements in Methods and Devices for Ultrasonic Imaging," and as described by the latter uses multiplex switches and shift registers to produce rectilinear images. There are several inherent limitations, such as relatively poor depth resolution, and these systems may not be capable of producing good images of the heart or good grey scale images. The phased array applied as a cardiac scanner is explained in "A New Ultrasound Imaging Technique Employing Two-Dimensional Electronic Beam Steering" by Thurstone and Von Ramm, Acoustical Holography, Vol. 5, 1974, Plenum Press, New York, pp. 249-259. The phased array utilizes time delay focusing and therefore has better resolution than in Fresnel focusing, but produces a sector scan image that is less pleasing and harder to manipulate.
It would be desirable to have a real time electronic scanner that will produce a rectilinear image and which uses time delay focusing for higher resolution. In the past there have been suggestions to achieve this goal, however, all of the suggested implementations require very large numbers of multiplex switches. For example, if each of 160 transducers is delayed by 32 different time delays, more than 5000 switches would be required. This is not feasible. The present invention has all of the advantages of Fresnel and phased array systems but is implemented with a reasonable number of multiplex switches.